Scooterotica

If we say that an exhaust port should measure 65% of the diameter of the cylinder and we know that the diameter is 69 mm, so that 65% would be 44.85 mm. How do we measure these 44.85 mm? Directly in the cylinder in a straight line from side to side of the port? Or we draw the port on a paper and then curve the paper into the cylinder? There is a big difference

straight across in a direct line from edge to edge

santacama wrote:If we say that an exhaust port should measure 65% of the diameter of the cylinder and we know that the diameter is 69 mm, so that 65% would be 44.85 mm. How do we measure these 44.85 mm? Directly in the cylinder in a straight line from side to side of the port? Or we draw the port on a paper and then curve the paper into the cylinder? There is a big difference

I believe that it applies to the curved measurement not the cord (straight line), you need to roll up a sheet of paper and tape insid the bore and then do a 'rubbing' with a pencil to get the outline to the port then roll it out flat and measure with a ruler.

I've been doing a lot of this lately

jonny snatchsniffer wrote:straight across in a direct line from edge to edge

I have a 225 barrel that measures 45mm across the Chord (straight line) which has snapped the bottom ring after less than 700 fairly gentle miles, ita's 55mm measured around the circumference which would equate to about 78% I think

Happy to learn more on this subject

coaster wrote:[Happy to learn more on this subject

Yes, a will be happy too.

55mm/69mm = 0,797; 79,7 %

if you have good wire rings you can go to 70% of bore or more with a good shape port but they will need changing more often
45mm with cast rings should be ok on a 70 mm bore, chamfer the port edges

the %age of port widths that you can safely go to and use is crucial and greatly dependant on the quality of parts used...
much depends on not only the quality of pistons and ring pegs but also on the quality of the rings and their spec ,the thinner they are and deeper section they are as well as their materials dictates how far you can safely go on port widths
also the shape of the ports themselves have a serious factor in the equation of performance/reliabity
whenever I widen an ex port ,i do it as a % age around the bore and not chord/straight across.

Straight across (the chord) or around the curve .

Which is the correct way

Only one way to find out. FIGHT !!!!!!!

sorry but I thought i,d made it plain....around the bore/curve

Taffspeed use to measure theirs straight across.